Android Question [Solved]How to save a list of arrays?

[RMarra]

I have a list and each object in the list is an array of objects. I'm using this to display in a B4XTable (which works fine). I need to store it when I exit the activity, so I can restore it the next time the activity starts. I tried Writelist but it doesn't like the array and (I think) it converts it to a string. I thought about writing it out as a CSV, but I don't see how without writing my own "convert-list-to-csv" routine. I don't really care how it is stored, as long as I can get it back correctly. Is there an easy way to do this?

 

[ronell]

use randomaccessfile library

randomaccessfile.writeobject

search the forum for samples

 

[Erel]

I recommend using B4XSerializator (RandomAccessFile library):

Dim ser As B4XSerializator
File.WriteBytes(File.DirInternal, "list", ser.ConvertObjectToBytes(MyArray))

Dim MyArray As List = ser.ConvertBytesToObject(File.ReadBytes(File.DirInternal, "list"))

Another option is to use KVS2.

 

[RMarra]

What is best practices on deciding where to put this file? In your examples you use DirInternal. I've tried this and a few other locations and either I get permission errors or can't find the file. I'd like to put the file on the SD card if possible.

 

[DonManfred]

Best practise is DirInternal.

I'd like to put the file on the SD card if possible.

To access a real SDCard you must use the ExternalStorage class.

 

[RMarra]

I can't get randomaccessfile to work for me. I'm only writing 5 rows containing a date, time and an integer. It's trying to create a huge file:

java.lang.OutOfMemoryError: Failed to allocate a 704739496 byte allocation with 6291456 free bytes and 253MB until OOM, max allowed footprint 8418576, growth limit 268435456

I could just write it out as text and parse it myself, but I'm trying to learn how to use new features.

Also, how can I see if there is a giant file orphaned out there?

Any other suggestions on how to do this? I've included the code snippet

 

[DonManfred]

Failed to allocate a 704739496 byte allocation

i dont think that randomaccessfile needs 704mb of ram to write 100 bytes of data.

The Problem must be somewhere other in your app.

 

[RMarra]

I wrote a test program to write out 1 row with B4xReadObject to position 1 and then immediately read it back and it works. I then write out 1 rows to position 2 and then immediately read it back and it works. I then try to reread #1 and it fails with (Exception) java.lang.Exception: java.io.EOFException: Unexpected end of ZLIB input stream.

                raf.Initialize(File.DirInternal, "InsulinHistory.dat",  False)
                Try
                    Dim rafRow(3) As String
                                        
                    rafRow(0) = "0"
                    rafRow(1) = "seconds=" & DateTime.GetSecond(DateTime.Now)
                    rafRow(2) = "2"
                    raf.WriteB4XObject(rafRow, 1)
                    rafRow = raf.ReadB4XObject(1)
                    Log(rafRow(1))
                        
                    rafRow(0) = "0"
                    rafRow(1) = "seconds=" & DateTime.GetSecond(DateTime.Now)
                    rafRow(2) = "2"
                    raf.WriteB4XObject(rafRow, 2)
                    rafRow = raf.ReadB4XObject(2)
                    Log(rafRow(1))
                        
                    rafRow = raf.ReadB4XObject(1)
                    Log(rafRow(1))
                Catch
                    Log(LastException)
                End Try

I've working on this for two days. Are there any other ways to do this? XML maybe?

 

[DonManfred]

how many items are in the list?
you should write the hole list/read the hole list in one step. If it is not too much items.

A good alternative is to use kvs2 and store the single items using a unique key one by one.

 

[emexes]

I am not set up right now to test this in B4A, but I think the problem might be that the Position parameter of the Read and Write methods is in bytes, not records.

So if your object ended up being saved as eg 120 bytes, then:

raf.WriteB4XObject(rafRow, 1) 'writes 120 bytes to file bytes 1..120
raf.WriteB4XObject(rafRow, 2) 'writes 120 bytes to file bytes 2..121, ie overwriting bytes 2..120 of the previous object

and then:

rafRow = raf.ReadB4XObject(1) 'reads bytes 1 (first byte of first object) and 2 (first byte of second object) and probably then careers off into the weeds

Imagine that the first four bytes are the length of the data to follow (or perhaps an object type identifier). After writing the first object, bytes 1..4 of the file would be 0116. After writing the second object, bytes 1..4 would be 0011 (the first 0 being the start of first object, and the 011 being the start of the second object). ReadB4XObject would likely (and reasonably) fail on reading from file position 1, since the data is incorrect. It should still read ok from file position 2.

The solution is to write the objects sequentially to the file, and likewise when reading them back. For each read/write operation, use:

for the Position parameter, eg:

raf.WriteB4XObject(rafRow, 0) 'writes first object to start of file (ie offset 0 bytes from beginning), at file bytes eg 0..119
...
raf.WriteB4XObject(rafRow, raf.CurrentPosition) 'writes second object to file, at file bytes eg 120..239
...
raf.WriteB4XObject(rafRow, raf.CurrentPosition) 'writes third object object to file, at file bytes eg 240..359

and then likewise for reading them back.

If you want to read them back in some arbitrary order (ie, random / direct access) then you will need to keep track of the CurrentPosition start address of each object. These would usually be kept in an array (properly, of Longs, but Ints will do the job for files < 2 GB, ie most everything except video and large databases). Next time you need this index array, you can either reconstruct it (by sequentially reading the file and noting CurrentPosition prior to each read) or you could save it to the end of the file followed by a WriteLong (or WriteInt?) of the CurrentPosition of that array.

 

[RMarra]

Ah! That makes sense. I'll give that a try. I'll adjust the code and see. Thanks!

 

[Erel]

Create a list with all the data that you want to save and save it with a single call:

File.WriteBytes(File.DirInternal, "list", ser.ConvertObjectToBytes(YourList))

 

[RMarra]

It's all working now. Thank you everyone. How do I mark this as solved?

 

[ronell]

Thank you everyone. How do I mark this as solved?

you can changed the title of the thread
[SOLVED]*insert title here*

 

[emexes]

It's all working now.

Out of interest: did you end up saving it as one list, or as individual elements? Was it an easy choice to make, or "six of one, half-a-dozen of the other"?